1. Field of the Invention
This invention is generally related to surface plasmon resonance (SPR) based devices. More particularly, the present invention is related to a surface modified optical fiber sensor of a wider refractive index range than heretofore possible.
2. Description of the Prior Art
Surface plasmon resonance is the oscillation of the plasma of free electrons which exists at a metal boundary. These oscillations are affected by the refractive index of the material adjacent the metal surface. Surface plasmon resonance may be achieved by using the evanescent wave which is generated when a p-polarized light beam is totally internally reflected at the boundary of a medium, e.g., glass, which has a high dielectric constant. A paper describing the technique has been published under the title "Surface plasmon resonance for gas detection and biosensing" by Lieberg, Nylander and Lundstrom in Sensors and Actuators, Vol. 4, page 299.
Illustrated in FIG. 1 of the accompanying drawings is a diagram of the equipment described in the Liegerg paper. A beam 1 of light is directed from a laser source (not shown) onto an internal surface 2 of a glass body 3. A detector (not shown) monitors the internally reflected beam 4. Applied to the external surface 2 of glass body 3 is a thin film 5 of metal, for example gold or silver, and applied to the film 5 is a further thin film 6 of organic material containing antibodies. A sample 7 containing antigen is brought into contact with the antibody film 6 to thus cause a reaction between the antigen and the antibody. If binding occurs, the refractive index of the film 6 will change owing to the increased size of the antibody molecules, and this change can be detected and measured using surface plasmon resonance techniques.
Surface plasmon resonance can be experimentally observed by varying the angle of the incident beam 1 and monitoring the intensity of the internally reflected beam 4. At a certain angle of incidence, the parallel component of the light momentum will match with the dispersion for surface plasmons at the opposite surface 8 of the metal film 5. Provided that the thickness of metal film 5 is chosen correctly, there will be an electromagnetic coupling between the glass/metal interface at surface 2 and the metal/antibody interface at surface 8 which results in surface plasmon resonance, and thus an attenuation in the reflected beam 4 at that particular angle of incidence. Thus, as the angle of incidence of beam 1 is varied, surface plasmon resonance is observed as a sharp dip in the intensity of the internally reflected beam 4 at a particular angle of incidence. The angle of incidence at which resonance occurs is affected by the refractive index of the material against the metal film 5, i.e. the antibody film 6, and the angle of incidence corresponding to resonance is thus a direct measure of the state of the reaction between the antibody and the antigen. Increased sensitivity can be obtained by choosing an angle of incidence half way down the reflectance dip curve where the response is substantially linear at the beginning of the antibody/antigen reaction, and then maintaining that angle of incidence fixed and observing changes in the intensity of the reflected beam 4 with time.
As the angle of incidence is changed, either by moving the light source or rotating the glass body, or both, the point on surface 2 at which the incoming beam 1 is incident moves. Because of inevitable variations in the metal film 5 and the antibody film 6, the angle of incidence at which resonance occurs changes as the point of incidence of incoming beam 1 moves, which, in turn, introduces a further variable factor into the measurement and thus makes comparison between the initial unbound state and the bound state of the antibody film 6 less accurate.
FIG. 2 shows a surface plasmon resonance sensor where the glass body is a prism 13 and a thin film 15 of metal is applied to its undersurface. Light 11 from a laser source is incident on the prism 13 where it is refracted at point 19 before entering the prism 13. The internally reflected beam 14 is likewise refracted at point 20 upon exiting from the prism 13. U.S. Pat. Nos. 5,064,619, 5,055,265, 5,047,633, 5,047,213, 5,035,863, 5,023,053, and 4,997,278 to Finlan, and U.S. Pat. No. 4,889,427 to VanVeen et al. describe prism-based SPR sensors.